As gas turbines age a certain amount of creep takes place in the hot and highly stressed parts. This creep results in extension of blades and discs which eventually will cause the blade tip clearance to become zero. Zero clearance results in a rub between the blade tip and the stationary parts of the turbine. Such minimum clearance becomes accentuated at particular periods of the cycle depending upon the type of turbine under consideration. For example, in a tapered gas turbine cylinder with the thrust bearing at the inlet end, as the rotor cools faster than the cylinder, the clearance reduces more on shutdown than normal. On the other hand, with an exhaust-mounted thrust bearing, the minimum clearance occurs during start-up.